A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared, any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common Typical trace lengths are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. Conventionally each trace records one seismic source activation, so there is one trace for each live source-receiver combination. In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2-D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3-D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
Controlled-frequency seismic sources are sources that are adjustable to produce a variety of different source signatures. This includes swept-frequency sources which emit a sinusoidal signal whose amplitude and frequency can be independently controlled. Examples of such include seismic land and marine vibrators. Swept-frequency sources may be preferable to conventional impulsive seismic sources for some geophysical applications, for example to minimize disturbance to nearby facilities by minimizing the peak emitted sound pressure from the sources. The ability to finely control the source frequency spectrum is also useful, as it allows the source to be optimized to meet the particular imaging challenge at hand. Conventionally, swept-frequency marine seismic sources are deployed at a depth such that the lowest non-zero-frequency spectral notch introduced by the surface ghost reflection does not occur within the frequency band of interest. For example, a source with a top frequency of 100 Hz would not be deployed deeper than about 7.5 m.
Although controlled-frequency seismic sources are quite useful in certain contexts, they are not without their challenges. For example, in order that a controlled-frequency source array should match an airgun array in output energy, the acoustic pressure at the faces of the individual sources is likely to be very high, well in excess of one bar. If deployed at too shallow a depth in the water (e.g. 7.5 m) cavitation can occur at the sources during the collection of a marine survey. The term “cavitation” refers to the formation of cavities (bubbles of vacuum or low-pressure vapor) in a fluid and the subsequent collapse thereof. The collapse generates a shock wave by implosion and the pressure wave(s) that result will be sensed and recorded by the hydrophones that are intended to collect the seismic survey data. The resulting pressure wave distorts or contaminates the recorded seismic signal making it more difficult to obtain a clear image of the subsurface.
Cavitation during a marine seismic survey alters the acoustic output of the array, rendering it less controllable; creates broadband noise which is environmentally undesirable; and can damage the sources themselves. One technique that has been used to minimize cavitation is to apportion the frequency band of interest among multiple sources, with each source in the array towed at an optimal depth for its frequency range. Even so, cavitation is still often present for the higher-frequency source elements in the array, which must be towed at the shallowest depths.
Heretofore, as is well known in the seismic acquisition and processing arts, there has been a need for a system and method that provides a method of reducing the likelihood of or preventing cavitation in controllable frequency marine seismic source arrays that does not suffer from known disadvantages. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
The description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.